Study of the adhered material to the cutting tools on dry turning of aluminium alloys.

De Agustina, Beatriz ; Rubio, Eva ; Marcos, Mariano 等

Abstract: In this study the relationship between the applied cutting parameters and the change to the tool geometry produced by material adhesion was analysed. Namely, a series of tests of dry turning of short duration (no longer than ten seconds) were carried out using a workpiece of aluminium UNS A97050-T7. Different cutting conditions were employed using uncoated based on WC-Co tools. The quantity of material adhered to the tool were measured by macroscopic techniques. As a first conclusion, it is possible to affirm that there is a higher material adherence using higher feeds.

Key words: Dry machining, Short cutting tests, UNS A97050 T7, Tool wear

1. INTRODUCTION

The aeronautical, aerospace and automotive industries use a larger number of aluminium and titanium alloys for the production of different elements that constitute the structural components of airships and aerospace vehicles. This is due to their excellent weight to resistance ratio.

However, these materials can commonly exhibit problems associated with the heat generated during de machining process that reduces their machinability and increases the temperature. This fact is especially injurious for the tool because of an increase of temperature can reduce its physical and chemical properties and, as a consequence of that, its life decreases (Nouari et al.,2003; Rubio et al.,2005; Sebastian et al.,2003).

Therefore, the use of lubricants and coolants during the machining constitutes an undesirable factor owing to environmental and economical factors. The growing social preoccupation towards environmental conservation has made it necessary to develop cleaner production technologies such as dry machining, in which no cutting fluids are employed. This increases tool damage, as these contact conditions become very severe.

Such a situation makes it necessary to look for new tool designs or, a cheaper alternative is to look for combinations of cutting parameters and types of tools that optimize the machining process, which allow the possibility to obtain pieces with a good dimensional precision and a high quality surface finish, to keep the cost as low as possible and, of course, to ensure secure conditions for workers and equipment (Rubio et al., 2005; Sebastian et al., 2003; Xie et al., 2003).

In this study, the investigation is focused on the different alterations of the tool geometry, due to the material adhered to the tool during the machining process. In this way, a series of tests of dry machining of short duration (no longer than 10 seconds) were carried out with a workpiece of aluminium UNS A97050-T7 under different cutting conditions (cutting speed and feed rate), using uncoated based on WC-Co tools. From the obtained results, a selection of tools was made to analyse the relationship between the applied cutting parameters and the quantity of material adhered to the tool. Furthermore, to verify if the results vary or not when the diameter of the tested bar diminishes; the tests were carried out twice.

2. METHODOLOGY

The present work is framed within a series of studies which involved different materials, types of tools and cutting conditions. The main steps of the methodology in which this work has been developed are (Agustina et al., 2007):

* Previous activities to the machining operations. These activities consist of the identification of the resources used and the preparation of the protocols both to calculate cutting parameter values and to register data and observations of the machining process.

* Turning tests. In each test a workpiece is mechanized during less than 10 seconds (short tests) under certain conditions of feed, cutting speed and depth of cut using both coated and uncoated cutting tools.

* Monitoring of the process. In order to have graphic documents that can be analysed after the process, all the turning tests described previously have been recorded by video and both the chips obtained and the inserts used in each test have been photographed with a camera of high resolution.

* Analysis of the tools with the reduction of the diameter. With the aim of determining if the material exhibits different behaviour according to the reduction of the diameter of the workpiece, the tests were carried out twice on the same bar so that the second series of tests began with a diameter considerably smaller than the first one.

* Analysis of tools. From the obtained results, a tool preselection was made. Then, selected tools were analysed employing both macroscopic and microscopic techniques. The former, using the taken macrographs and a profile projecter that allows measuring the quantity of the adhered material and the latter, by means of techniques of Scanning Electron Microscopy (SEM) and Energy Dispersive Spectrometer (EDS) in order to verify the alterations of the tools geometry.

3. APPLICATIONS

For this study, the workpiece used for the turning tests was a cylindrical bar with a diameter of 70.7 mm and length of 60.5 mm of UNS A97050-T7 aluminium alloy.

The cylindrical bar was horizontally dry turned on an EmcoTurn 120 CNC lathe equipped with an EMCO Turn 242 numerical control. The cutting conditions are collected in Table 1.

Concretely, cutting speeds from 40 m/min (0.66m/s) up to 170 m/min (2.83 m/s) and feeds from 0.05 mm/rev up to 0.30 mm/rev. Cutting depth was maintained at 1 mm in all the tests.

[FIGURE 1 OMITTED]

The cutting speed and feed rate are expressed in units that are usually employed in manufacturing workshop. Although they are not International Systems units (S.I.), they give a more intuitive idea of the values used.

Uncoated WC-Co (manufacturer reference GARANT DCMT 11T304CU703326236 SS) cutting tools were employed for the tests.

To observe the machining tests carried out, photographs and videos of the tools and the resulting chips were systematically taken during the tests using a Sony Cybershot DSC-P100 digital camera of high resolution.

For quantifying the adhered material on the tools a profile projecter TOPCON VP300D (Figure 1) was used and for the SEM/EDS analysis, a Scanning Electronic Microscope, called Quanta 200, which has a system of Energy Dispersive Spectrometer was also used.

4. RESULTS

It can be observed that for all the values of cutting speed applied the higher the feed was, the larger the quantity of material that was adhered to the rake face of the tool . Figure 2 shows the tools used for the combination of the cutting speed of v= 170 m/min for both, f=0.2 and 0.3 mm/rev tested.

In addition, at low feeds of 0.05 and 0.10 mm/rev, for cutting speeds from 65 to 170 m/min, a reduced quantity of material is adhered on the rake face of the tool. The inserts used at the cutting speed of 65 m/min for feed rate from 0.10 to 0.30 mm/rev, and at the feed of 0.20 mm/rev for cutting speeds from 65 to 125 m/min, were selected for measuring the maximum thickness ([t.sub.max]) of the material adhered to the edge tool by the profile projecter. The results are given in the next Table 2.

It can be seen that for a cutting speed of 65m/min, [t.sub.max] is at the maximum feed of 030 mm/rev. Meanwhile, for a feed of 0.20 mm/rev, [t.sub.max] is at the minimum cutting speed of 65 mm/rev.

Deeper analysis by Scanning Electron Microscopy (SEM) and Energy Dispersive Spectrometer (EDS) has been carried out in order to obtain with more accuracy the quantity of the material adhered and its compositional characteristics.

[FIGURE 2 OMITTED]

Finally it was noted, in general, that no difference was observed in the material adhered to the tool as the centre of the bar was approached, in spite of the existence of a microstructural differential at the centre of the bar with respect to its periphery. In such a way, the centre of the bar possesses a more homogeneous structure and higher hardness, as expected given the results carried out by other researchers (Ozcatalbas, 2003), as the centre of bar is approached, it should be reduced the quantity of material adhered to the tool especially at high cutting speeds.

5. CONCLUSIONS

A larger quantity of material is adhered to the tool rake face at the higher applied feeds . In these cases the alteration of the tool geometry can degrade the efficiency of cutting tool and surface finish of the workpiece.

It is not possible to establish a relationship between the cutting parameters applied and the maximum thickness of material adhered to the edge tool. So it is proposed that the same experimental procedure is conducted using different cutting parameters concentrating in smaller range of values.

Finally, no difference was observed in the quantity of material adhered to the tool on reducing the diameter of the tested bar. To evaluate such an effect, it is recommended to carry out a series of tests using bars of considerably different diameters maintaining the identical test methodology.

6. ACKNOWLEDGMENTS

Funding for this work was provided in part by the Spanish Ministry of Education and Science (Directorate General of Research), Project DPI2005-09325-C02-02.

7. REFERENCES

Agustina, B.; Rubio, E.M.; Sanz, A., Domingo R. (2007) A classification of the UNS A97050-T7 aluminium alloy chips in short duration tests under dry cutting conditions, Proceedings of the MESIC-CISIF'2007, SIF, In press, Madrid, July 2007. Nouari, M.; List, G.; Girot, F.; Coupard, D. (2003). Experimental

analysis and optimisation of tool wear in dry machining of aluminium alloys. Wear 255 (7-12) August-September 2003, 1359-1368.

Ozcatalbas,Y.; (2003). Investigation of the machinability behaviour of Al4C3 reinforced Al-based composite produced by mechanical alloying technique.Composites Science and Technology, 63(1) August 2003, 53-61.

Rubio, E.M; Camacho A.M.; Sanchez-Sola, J.M.; Marcos, M. (2005). Surface roughness of AA7050 alloy turned bars. Analysis of the influence of the length of machining. Journal of Materials Processing and Technology, 162-163C May 2005, 682-689.

Sebastian, M.A.; Sanchez, J.M.; Rubio, E.M.; Carrilero, M.S.; Diaz, J.E.; Marcos, M. (2003). BUE and BUL formation mechanisms in dry cutting of AA7050 alloy, Proceedings of the 14th International DAAAM Symposium, "Intelligent Manufacturing & Automation: Focus on Reconstruction and Development". B. Katalinic, 125-126, Viena, October 2003.

Xie, X.; Yan X.; Ding, F.; Zhang, S.; Chen, M.G.; Chang, Y.A. (2003). A study of microstructure and microsegregation of aluminum 7050 alloy. Materials Science and Engineering A355, August 2003, 144-153.

Table 1. Cutting conditions.

v (m/min) 40 65 85 125 170
f (mm/rev) 0.05 0.10 0.20 0.30
p (mm) I

Table 2. Maximum thicknesses ([t.sub.max]) of the material adhered
to the edge tool at different cutting parameters.

v (m/min) 65 65 65 85 125
f (mm/rev) 0.10 0.20 0.30 0.20 0.20
[t.sub.max] (mm) 0.292 0.270 0.378 0.234 0.265